An auxiliary bearing is used to support a shaft in the event of a failure of an associated magnetic bearing. When a magnetic bearing fails, the rotating shaft comes in contact with the inside of the inner ring of the auxiliary bearing, and this leads to an acceleration of the inner ring of the bearing. These contacts affect the shaft, resulting in the frictional forces. The shaft starts to whirl in the clearance between the shaft and the inner ring of the auxiliary bearing. The friction forces are directed to increase the whirling speed until the auxiliary bearing has accelerated almost to the speed of the rotating shaft. The resulting whirl speed causes a centrifugal force which can be 30-50 times the centrifugal force associated with the shaft rotating steadily about its center axis without any whirling motion. The limited contact area and cooling area often also cause temperature problems.
EP-0 499 310 A1 discloses a magnetic bearing shaft assembly having a back-up bearing to support the shaft in the event of a failure of the magnetic bearing. If the bearing portion of the back-up bearing includes a ball race, the radially outer raceway of the bearing is secured to an anchorage, such as a housing for the assembly. A low stiffness damper member in the form of an annular shaped body of resilient material is also included in the back-up bearing at a position between the bearing portion of the bearing and the anchorage. When the magnetic bearing fails and the shaft is being brought to rest by being in contact with the back-up bearing, the resilient body is in shear and the body has a lower stiffness than otherwise would be the case. Thus, any tendency for the back-up bearing and/or the anchorage to be damaged while the shaft whirl is being brought to rest is reduced because the provision of the low stiffness damper causes the shaft to have a low natural frequency of oscillation. The stiffness damper member constitutes a crucial part of the assembly.
A need exists however for a way of limiting the whirling movement of the rotatory shaft after a failure of the magnetic bearing has occurred. The problem associated with such whirling movement is particularly accentuated in the case of a vertical shaft.